This application claims the priority of Korean Patent Application Nos. 2003-89070, filed on Dec. 9, 2003 and 2004-69556 filed on Sep. 1, 2004 in the Korean Intellectual Property Office, the disclosures of which are incorporated herein in their entirety by reference.
1. Field of the Invention
The present invention relates to an optical network terminal (ONT) and a wavelength division multiplexing (WDM) based optical network having the same, and more particularly, to a loop-back ONT using a Febry Perot laser diode (FP-LD) as an active optical modulator, not as an optical source, in a WDM based optical network and a WDM based optical network having the same.
2. Description of the Related Art
It is expected that the integrated triple play service of voice, data, and digital broadcasting service will be a major stream in communication services within several years. However, current digital subscriber line (DSL) technologies using a unshielded twisted pair (UTP) or cable modem termination system (CMTS) technologies using a hybrid fiber coaxial (HFC) cable would not guarantee sufficient bandwidths and transmission quality required to provide subscribers with such a high quality service. To solve this problem, a fiber to the home (FTTH) technology connecting optical fibers up to user's premises is being widely investigated and developed.
The most important point in developing the FTTH technology is how to obtain an optical signal transmission method which satisfies cost effectiveness and mass-productability.
On the other hand, an optical subscriber network can be classified into a passive optical network (PON) type and an active optical network (AON) type. The PON type is being developed in a variety of forms, such as ATM-PON, B-PON, G-PON, and E-PON, while the AON type is being developed in such a form that local networks composed on Ethernet switches are interconnected via optical fibers.
According to the above conventional technologies, a single wavelength is used to transmit data in each transmission direction via an optical transmission line. Therefore, such a transmission method has limitation in providing a broad bandwidth over 100 Mbps required to guarantee transmission quality for future user's needs. To overcome such a shortcoming, there have recently been efforts to introduce a wavelength division multiplexing (WDM) technology to the FTTH subscriber network in a variety of methods.
Until now, various optical transmission types have been proposed to implement such a WDM based FTTH network. Roughly, they can be classified into two types: a first type that a light source of a predetermined wavelength is installed in the ONT for upstream transmission; and a second type that the ONT has a modulator instead of the light source to modulate the light beam from the central office and then send it back to the central office. The second type is often called a loop-back type.
In the first type, typically, different wavelengths are allocated to different users connected to a particular optical line terminal (OLT). Therefore, N optical network terminals (ONTs) for outputting different wavelengths (e.g., λ1˜λN) of optical signals must be provided for N users. This causes additional workloads for producing, installing, and managing the N different ONTs, which will be a more serious problem when more and more ONTs are installed.
To solve such a wavelength allocation problem, another transmission type has been proposed. In this type, a light source having no predetermined wavelength is provided to each user, and an upstream wavelength is determined based on the light beam from the central office to ONT. For this purpose, various implementation methods have been proposed.
In an implementation method, the central office transmits an incoherent wideband light beam to the ONT having super luminescence light emission diode (SLED) or erbium doped fiber amplifier (EDFA), and the transmitted wavelength is divided by the WDM de-multiplexer near the ONT and then injected into a modulator (e.g., a Febry-Perot laser diode) of the ONT to lock a wavelength of the operation modes of the FP-LD with the wavelength of the injected light. Then, the FP-LD is modulated with the upstream data under the locking state.
According to this method, the EDFA is a most effective wideband light source considering optical power of a master light beam at OLT for an injection locking of the ONT, which will be described below. However, the EDFA is still very expensive so that it does not satisfy the cost-effectiveness as required for the subscriber access network. Furthermore, this method has a limitation in a data modulation speed due to its innate optical characteristics, so that a high-speed modulation over 1 Gbps would be difficult to implement.
Another implementation method to get around the wavelength predetermination problem is to eliminate light source in the ONT. This loop-back ONT is preferable from the viewpoint of cost-effectiveness because a light source with a predefined wavelength is not required so that mass-production can be easily achieved, and the resulting ONT can be conveniently installed in the user's premises.
Typically, a conventional loop-back ONT uses a Mach-Zehnder interferometer type modulator to modulate the light beam from the CO (i.e., downstream optical signals) with data to be transmitted from a user in an entire or partial time interval to produce upstream optical signals and then transmit them to the CO.
However, a conventional loop-back ONT has shortcomings that the Mach-Zehnder interferometer is expensive and has a large optical power losses (e.g., about 7˜10 dB, typically), and furthermore the downstream optical signal, which has experienced a lot of losses on the downstream path from the CO to the user, could not be reused to produce the upstream optical signal without amplification and then retransmit it to the CO. In addition, it would be also expensive to employing a conventional loop-back ONT because an optical amplification still requires considerable expense.